It is generally known that internal combustion engines produce exhaust that contains a variety of federally regulated constituents. For example, the various oxides of nitrogen (NOx) such as NO, NO2, and the like are regulated. To effectively reduce the NOx concentrations in the exhaust stream of lean burning engines including natural gas, gasoline, and diesel engines, the use of selective catalytic reduction of NOx with a hydrocarbon in oxygen rich conditions (lean NOx catalyst systems) is actively being pursued.
Unfortunately, many catalysts or catalytic systems that have been utilized for lean burn engines to date suffer from low removal of NOx, inadequate catalyst durability, low thermal stability and a limited temperature window of operation. For example most commercially available lean NOx catalytic systems for lean burn engines achieve less than 20% NOx reduction as a passive system and possibly up to 40% for active systems (i.e. when supplemental hydrocarbon reductant is introduced into the exhaust stream). In addition, most commercially available lean NOx catalytic systems also are subjected to sulfur poisoning, from the minimal amounts of sulfur found in many fuels and lubricants, resulting in low catalyst durability.
Such active catalytic systems involving lean NOx catalysts require a sufficient concentration of hydrocarbon (HC) species to be present. In other words, in combustion exhaust purification systems having an oxygen environment above 4% concentration, some type of reducing agent, usually a hydrocarbon compound such as diesel fuel, must be introduced into the exhaust in order to achieve acceptable reduction levels of NOx compounds. However, these and other such exhaust gas after treatment systems introduce fuel or a hydrocarbon source from a secondary supply system that tends to reduce the overall fuel efficiency of the engine system or necessitate maintaining the secondary supply system.
An exemplary exhaust gas after treatment system employing an ether such as triglyme that is added to a hydrocarbon source and injected into the exhaust system is described in U.S. Patent Application Publication No. 2014/03181102 (“the '102 publication”), published Oct. 30, 2014. The exhaust gas after treatment system described in the '102 publication includes a dosing system with tank of dosing fluid that is injected into the exhaust stream. This dosing fluid include a fuel such as diesel and activator material that is an oxygenated hydrocarbon. In a particular example, the activator material is the ether, triglyme. This triglyme is mixed with the diesel and maintained in a separate tank from the fuel tank for the engine.
While the system of the '102 publication may be configured to reduce NOx, such systems may have drawbacks. For example, maintaining the separate dosing fluid is bothersome for the end user and represents an added expense of operating the engine. If the dosing fluid is not maintained, the exhaust may emit NOx at levels that exceed federal guidelines and may result in fines. In addition, if the dosing fluid is not maintained, the catalytic system in the exhaust system may be damaged.
Accordingly, there is a need for improved exhaust gas after treatment system to address the problems described above and/or problems posed by other conventional approaches.
It will be appreciated that this background description has been created to aid the reader, and is not a concession that any of the indicated problems were themselves known previously in the art.